The present invention relates to a process for bleaching a fibrous raw material such as wood pulp, cellulose, used paper and/or a mixture thereof using hydrogen peroxide or dithionite.
Heavy metals and salts thereof catalyze the decomposition of chemical bleaching agents (e.g. hydrogen peroxide or dithionite). In order to minimize the losses of the bleaching agent and to enable the carrying out of the bleaching process in a batch having a high heavy metal content, chelate forming agents are added in actual practice.
In principle all chelate forming agents can be used which form a complex of suitable stability and which are not attacked by the bleaching agent in the course of the bleaching process. Generally organic compounds are used. In the paper and textile industry particularly, salts of nitrilo-tri-acetic acid (NTA), ethylene diamine tetraacetic acid (EDTA) and diethylene triamine pentaacetic acid (DTPA) are used due to the excellent stability of the complexes formed thereby. Ions of the metals iron, cobalt and manganese, which exhibit a particularly unfavorable effect during the bleaching process by causing catalytic decomposition, are particularly efficiently bound by the polyvalent DTPA.
Weaker chelate forming agents, such as citric acid, tartaric acid, saccharic acids, polymeric acrylic acid derivatives or lower polyoxycarboxylic acids, do not form generally complexes of higher stability. In order to protect bleaching agents from decomposition induced by heavy metals, said agents are added in extremely high amounts. For this reason the use of said bleaching agents according to the state of prior art is not economically acceptable, although these bleaching agents are practically quantitatively decomposed by aerobic or anaerobic procedures in conventional sewage purifying equipment.
The disadvantage in using the above-mentioned EDTA and DTPA as well as also the phosphonic acid thereof (e.g. diethylene-triamine-pentamethylene phosphenate), resides in the fact that these compounds can be decomposed by biological means only in a very difficult and circumstantial manner. Basically an accumulation of the precipitated heavy metals in the environment and the risk of subsequent release thereof (e.g. from sediments in running waters) can not be avoided.
In German Application No. 37 39 655 an alkaline peroxide-containing bleaching agent is disclosed which comprises a silicate-type ion-exchanger modified with an alkali carbonate or alkali hydrogen carbonate additive. This process in said to constitute a solution of the problem, namely providing a bleaching process without requiring addition of alkali hydroxide or, at most, the addition thereof in only a small amount; and without the addition of water glass or, at most, the addition thereof only in small amount; and without bleaching addition of a complex forming agent, or the addition thereof only in a small amount.